The present invention relates to computed tomography (CT) imaging apparatus; and more particularly, to a method for reducing streaking artifacts caused by limited x-ray photons at certain view angles.
In a computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an x-y plane of a Cartesian coordinate system, termed the "imaging plane". The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile. The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a "view" and a "scan" of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. The scan data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
A number of factors can contribute to the production of artifacts in the reconstructed image. One of these factors is insufficient x-ray flux due to attenuation of the x-ray beam by the object. For example, bones in the shoulders of a medical patient will highly attenuate x-rays directed horizontally through the chest of the patient and the resulting low x-ray counts at the detectors introduce uncertainty into the acquired view. This uncertainty is manifested as horizontal streaks in the reconstructed image. This uncertainty cannot be reduced by increasing overall x-ray flux density, because this would overload other detector channels receiving less attenuated beams and increase patient x-ray dosage. The streaking cannot be filtered from the image using conventional filters, because such filters reduce image resolution and they are ineffective due to the severity of the streaking artifacts.